This photograph from the International Space Station highlights a late-summer “whiting event” visible across much of Lake Ontario (one of North America’s Great Lakes). Such events commonly occur in late summer and are caused by changes in water temperature, which allows fine particles of calcium carbonate to form in the water column. Increased photosynthesis by phytoplankton and other microscopic marine life can also reduce the amount of carbon dioxide in the water column, changing the acidity and allowing calcium carbonate to form. These particles of calcium carbonate cause the characteristic lightening (“whiting”) of the water color observed.

Lake Ontario—like the Great Lakes Erie, Huron, and Superior—is roughly divided between the United States and Canada. The USA side of Lake Ontario has its shoreline in the state of New York, while its Canadian shoreline lies within the province of Ontario. The city of Kingston, Ontario, is visible near the Saint Lawrence River outflow from the lake. Several other landscape features of New York State are visible in the image, including the Finger Lakes region to the west of Syracuse. To the northeast of Syracuse, the dark wooded slopes of the Adirondack Mountains are visible at image upper right. Patchy white cloud cover obscures much of the land surface to the west of Lake Ontario.

Editor’s Note: A previous version of this story identified the discoloration of the water as a bloom of phytoplankton or harmful algae. Such events can produce stunning color changes, as shown here. However, scientists working on the water in the area provided information to verify that it was instead a whiting event.

Astronaut photograph ISS036-E-35635 was acquired on August 24, 2013, with a Nikon D3S digital camera using a 50 millimeter lens, and is provided by the ISS Crew Earth Observations experiment and Image Science & Analysis Laboratory, Johnson Space Center. The image was taken by the Expedition 36 crew. It has been cropped and enhanced to improve contrast, and lens artifacts have been removed. The International Space Station Program supports the laboratory as part of the ISS National Lab to help astronauts take pictures of Earth that will be of the greatest value to scientists and the public, and to make those images freely available on the Internet. Additional images taken by astronauts and cosmonauts can be viewed at the NASA/JSC Gateway to Astronaut Photography of Earth. Caption by William L. Stefanov, Jacobs at NASA-JSC.